Ask New Jersey Gov. Thomas H. Kean what he thinks about President Reagan's "New Federalism" and he offers the standard Republican response.

"I support the thrust of the New Federalism," said Kean. "I support what the president is trying to do. I believe we can do things cheaper and more efficiently than the federal government."

Then ask Kean, who took office nearly a year ago, about the specifics.

What about the states taking over the government's main welfare program, aid to families with dependent children (AFDC)?

"I happen to feel strongly that maintaining programs for people in need should be done at the national level," he said. "We have been much more humane than some other states, and we would become a magnet for the poor."

What about the federal government taking over Medicaid, where New Jersey provides a range of optional services that might be dropped?

"I would want a floor on those services," Kean insisted. "I don't think we're outrageously generous. I don't think that providing eyeglasses for old people is something we shouldn't be doing."

What about federal subsidies for running buses and trains, which have been cut by a third and are slated for elimination?

"I'm concerned about the tragedy and lack of understanding in that federal policy," Kean said. "Every civilized country I know provides operating funds for mass transit."

And what of Reagan's plan to phase out federal funding for these and other programs after seven years?

"They cannot give us the resources for a couple of years and then phase it out," he said. "Otherwise, somebody's going to get hurt."

Despite the president's assurance that there will be "no winners and no losers" under New Federalism, the arithmetic all depends on where you live. A poor state like Mississippi has only 71 percent of an average state's income available for taxation, while Alaska has 215 percent of the average income.

Some states have more to lose because they provide higher health and welfare benefits than their neighbors. In 1980, New Jersey spent an average of $1,118 on each Medicaid patient, far more than the $646 spent by Oregon, but considerably less than the $1,985 paid out by New York. A national program probably would set the level of benefits somewhere in between, which would be a windfall for Oregon but a sizable cut for the big-spending states.

A Medicaid patient in New Jersey is eligible for prescription drugs, dentures, eyeglasses and artificial limbs. He can get a wheelchair, bandages or braces. He can check into an intermediate-care nursing home or visit a podiatrist, optometrist, psychologist or chiropractor. All this comes to 41 percent of the state's cost, and much of it could be dropped under a national program.

If the states had to take over AFDC, New Jersey again would be saddled with a bigger burden, for it pays welfare families $414 a month, more than all but a dozen states. Mississippi, the least generous state, pays $96 a month. Unlike Medicaid, these payments do not rise with inflation, and even New Jersey has raised them just four times since 1970. A monthly welfare check here, which once paid for 88 percent of a low standard of living, now covers little more than half.

There was a time, Kean recalled, when his state had the best roads in the country, when it provided ample care for all the poor people who migrated there from the South, when it built its own highways and tunnels without the help of big federal grants.

Now, he says, just when New Jersey's economy is ailing and its aging roads and sewers have fallen into disrepair, Washington is heavily subsidizing growth in the energy-rich southern states. New Jersey ranks 45th in aid from Washington, Kean says, because the federal formulas are unfairly tilted toward the Sun Belt.

This, critics say, is the problem with the New Federalism: It doesn't consider the vast regional differences in wealth, in cost of living, in natural resources.

As Assembly Speaker Alan Karcher put it: "New Jersey wouldn't have this problem if we had oil wells."